This invention relates to an image reading apparatus for use in a digital copying machine, facsimile machine or its equivalents.
Generally, an image reading apparatus such as an image scanner has been known in which a multitude of photoelectric conversion elements such as CCDs are arranged in a highly dense manner to read an image of an original document. In such image reading apparatus, light reflected from the original image is guided to the photoelectric conversion elements via an optical system to obtain data concerning the intensity of received light (hereinafter merely referred to as "light intensity data").
To obtain accurate light intensity data on the original image, a reference plate of white color having a standard brightness (hereinafter merely referred to as a "shading correction plate" or "reference plate") is employed to compensate shading resulting from irregularities of illuminated light and variations of sensitivity to light of the photoelectric conversion elements.
Specifically, the image reading apparatus reads the intensity of light reflected from the shading correction plate to obtain reference data on the shading correction plate, and then the light intensity data on the original image is corrected on the basis of the thus obtained reference data on the shading correction plate. This correction is called "shading correction".
The above conventional image reading apparatus has suffered from the following drawback. If the shading correction plate or the optical system has been smeared with dusts, dirt, and the like, accurate light intensity data on the shading correction plate, or data for shading correction cannot be obtained, and accordingly, a precise shading correction is not executable. To solve such problem, an image reading apparatus has been proposed which has the capability of judging whether the cause for producing inaccurate data is due to smearing of the shading correction plate or of the optical system (Japanese Unexamined Patent Publication No. HEI 6-78147).
However, the image reading apparatus proposed in the above patent publication cannot perform an accurate image reading unless the cause for occurrence of inaccurate data for shading correction is removed, i.e., unless the optical system and/or the shading correction plate has to be cleaned, even though a service person is informed of the judgment result that the cause is due to smearing of the optical system or of the shading correction plate. That is, regardless of the content of the judgment result, the optical system and/or the shading correction plate must be cleaned to perform an accurate image reading, which is cumbersome for a service person and impairs workability of the image reading apparatus.